An article (link to the file in PDF format) published in the journal “Astronomy & Astrophysics” reports the detection of a pair of supermassive black holes in the galaxy NGC 7727. A team of researchers used ESO’s VLT to conduct the observations that led to a discovery that broke two records in this field. The two supermassive black holes are the closest pair discovered so far at about 89 million light-years from Earth and are the closest to each other, as their distance was estimated to be around 1,600 light-years.
The presence of a supermassive black hole at the center of a galaxy is normal but when two galaxies merge, they end up with two of these objects. Despite their enormous masses and gravity, it can take many millions of years for two supermassive black holes to attract enough to collide and merge as well. The consequence is that there are cases where astronomers discover these pairs, and normally the distance between them is several thousand light-years. This time, however, astronomers discovered a relatively close pair.
Galaxy NGC 7727 exhibits some anomalous characteristics compared to a normal spiral galaxy with barely visible arms. Previous studies already concluded that it’s the result of a galaxy merger that took place about a billion years ago, so it was a candidate to host two supermassive black holes. However, discovering their presence is not easy when their activity is not enough to generate strong electromagnetic emissions or when those emissions are shielded by gas and dust.
Astronomer Karina Voggel of the Strasbourg Observatory, France, lead author of the new research, learned to use the MUSE (Multi-Unit Spectroscopic Explorer) instrument installed on the VLT (Very Large Telescope) in Chile as a student at ESO. This spectroscope played a crucial role in identifying the pair of supermassive black holes in the galaxy NGC 7727 together with data from observations conducted with the Hubble Space Telescope.
The pair of supermassive black holes that were previously closest to Earth is approximately 470 million light-years away. The pair in NGC 7727 is about 89 million years away, a plus to get more information about them. The image (ESO/Voggel et al.; ESO/VST ATLAS team. Acknowledgement: Durham University/CASU/WFAU) shows on the right NGC 7727 seen by the VLT and on the right its central part seen by the VLT’s MUSE instrument with the two supermassive black holes.
Also thanks to the MUSE instrument’s sensitivity, the researchers were able to measure the force of gravity exerted on the stars around them and thus estimate their masses. The supermassive black hole at the center of NGC 7727 has a mass estimated to be around 154 million times the Sun’s while the other has a mass estimated to be around 6.3 million times the Sun’s.
The two supermassive black holes seem to be at an advanced stage in their approach because the distance between them is “only” 1,600 light-years, less than half of the previous record. According to Professor Holger Baumgardt of the University of Queensland, Australia, one of the authors of the study, they will merge within 250 million years. Typically, pairs of objects of this type have distances of several thousand light-years between the two of them but this depends a lot on when the galaxy merger that brought them together took place.
ESO astronomer Steffen Mieske, Head of ESO Paranal Science Operations, another of the study’s authors, stated that the detection of a pair of supermassive black holes is just the beginning because ESO is building the ELT, a next-generation telescope that could provide detections of far more distant pairs. According to Karina Voggel, there are other hidden pairs whose detection could increase the number of known supermassive black holes in the local universe by 30%.
Increasingly sophisticated instruments and increasingly refined techniques are helping astronomical research, including that of supermassive black holes. Observing these extreme objects will help to better understand limit conditions and their influence on the evolution of galaxies and in particular on star formation.